Electrodes have been used to stimulate contraction of the heart or to reverse certain life-threatening arrhythmias, where electrical energy is applied to the heart via the electrodes to return the heart to normal rhythm. Electrodes have also been used to sense and deliver pacing pulses to the atrium and ventricle. Cardiac pacing may be performed by a transvenous method or by electrodes implanted directly onto the epicardium. For transvenous pacing systems, a lead having an electrode is positioned in the right ventricle and/or in the right atrium through a subclavian vein, and the proximal electrode terminals are attached to a pacemaker which is implanted subcutaneously.
Some lead designs have “floating” electrodes or electrodes which are not attached to the endocardial wall of the heart. The floating electrodes lay in the blood pool or against the endocardial wall of the heart and the electrode may move slightly within the heart. Since the location of floating electrodes is not fixed with respect to the endocardial wall, the electrical performance of these electrodes varies and is generally less than optimal. Both the electrical sensing capability as well as the pacing delivery capability of such electrodes are suboptimal. The pacing parameters of such a floating electrode are also suboptimal. In addition, the floating electrodes can require increased voltage which unnecessarily drains the battery.
As an alternative to floating electrodes, leads have been provided with passive fixation elements that affix the electrode to the endocardial wall over time. With passive fixation elements, it can be difficult to determine whether the lead will affix in the location at which it is implanted.
Active fixation elements, such as a helix, have also been provided with distal ends of leads which allow a lead to be affixed to the endocardial wall. A lead with an active fixation element may be inserted into a patient by a number of different medical procedures. The less invasive or traumatic the procedure, the more desirable is that procedure. For example, although the electrodes may be inserted by open chest surgery, the delivery of the electrode through catheterization techniques through arteries or veins is much more preferred. The difficulties involved with passing a sharp element through the vasculature of a patient can be readily appreciated, especially where the path can be tortuous or partially clogged with deposits. To avoid damage to the patient, the GUIDANT™ Sweet-Tip™ Model 4269 and the GUIDANT™ Sweet Pico Tip bipolar endocardial leads provide a mannitol cap over the helical element in the lead. The mannitol cap provides a protective cover for the helical element which prevents the point of the helical element from scraping or puncturing interior walls of the vasculature or other tissue during introduction of the element to the patient. The physician must wait for the mannitol cap to dissolve after the lead is implanted into the heart before the helical element can be affixed to tissue of the patient.
Other formats for delivering helical or barbed elements to secure an electrode into contact with appropriate tissue have utilized securing elements which are in a retracted position within the end of the delivered electrode. The retracted element is rotated and advanced into an exposed and operative position after positioning the distal end of the electrode element within the heart of a patient. Advancement and exposure of the retracted element may be effected by winding or screwing the helical element with a stylet disposed within the lead, or with the pin cap. As the stylet or the cap is rotated, however, the active fixation element may jump out of the end of the lead due to friction, for example, from a buildup of tissue or blood within the lead. Unexpected movement of the helix leads to potential tissue damage, and longer implant times. In addition, it is difficult for the physician to determine how many turns to the stylet or pin cap are necessary to fully advance the helix from the lead.